Measurement of electrical resistivity beneath the seafloor has assumed an important role in hydrocarbon, e.g., oil or natural gas, exploration and reservoir assessment and development. Seismic methods had traditionally been used for such purposes, however, the results could be ambiguous. Several electromagnetic methods have been developed for mapping sub-seafloor resistivity variations. See, for example, U.S. Pat. No. 5,770,945 of Constable (magnetotelluric methods), U.S. Pat. No. 6,522,146 of Srnka (controlled EM source), International Publication No. WO 03/048812 of MacGregor and Sinha (controlled EM source), and International Publication No. WO 01/57555 of Rees (controlled EM source). The disclosure of each of the identified patent documents is incorporated herein by reference. Controlled EM source (CSEM) methods are well known in the art and have become almost routine for mapping of electrical conductivity of the seafloor in very shallow to deep ocean water, achieving seafloor penetration depths as great as 30 km in 5 km of water. Such methods have provided significant economic savings in terms of avoiding the costs of drilling test wells into sub-seafloor structures that do not contain economically recoverable amounts of hydrocarbon. However, to date, applications of these methods to hydrocarbon reservoirs have been limited to identification and mapping of hydrocarbon-filled layers.
Once it has been located and tapped, it is important to monitor changes in the hydrocarbon reservoir during gas/oil extraction for optimal production. One existing technique involves the drilling of discrete monitoring wells, which, in addition to being expensive and time consuming, are incapable of providing a comprehensive image of reservoir geometry due to the small number of sampling locations. Another technique involves the placement of sensors in the extraction well or “well-logging”, in which the drill string is withdrawn, instruments are inserted for measurement then withdrawn, and the drill string is replaced. Other existing methods include repeated, or time lapse, seismic surveys, also known as 4-dimensional seismic modeling. Seismic surveys involve the use of air gun arrays to generate loud seismic pulses at the surface which are reflected by boundaries between the layers of rock and the hydrocarbon reservoir. The seismic method measures acoustic properties of the sub-seafloor structure, which are less well correlated with actual hydrocarbon content than is electrical resistivity. In addition, due to the hazards to those who operate the air gun or are located nearby and the damage to or disturbance of marine life in a wide area around the pulses, continuous or repeated seismic surveying is both undesirable and impractical for extended monitoring applications. Further, borehole seismic surveying involves expensive well down-time.
Accordingly, the need remains for a system and method for economically monitoring a reservoir over time for enhanced recovery of hydrocarbon from the reservoir with minimal damage to the environment from the monitoring technique.